The invention relates to an air spring having two end members, namely, a cover and a roll-off piston between which a flexible member is clamped so that the end members are movable relative to each other in the longitudinal direction of the air spring The air spring further includes an electrically conductive helix aligned in the longitudinal direction of the air spring. The helix is so mounted in the air spring that it changes with respect to its length when the end members move toward each other in the longitudinal direction of the air spring.
It has long been known to build level control systems having air springs into motor vehicles. These systems afford the advantage that the level of the vehicle can be held constant independently of its state of loading. The level control system requires elevation sensors to control the level and these sensors are assigned to the air springs. The elevation sensors can be mounted either outside of the air springs or can be integrated therein. An integration of the elevation sensors in the air springs affords the advantages that the elevation sensors are protected against mechanical loads and, furthermore, do not have to be mounted as separate components on the motor vehicle.
German patent publication 4,413,559 discloses an air spring having an integrated elevation sensor. The elevation sensor comprises an electrically conductive helix which is integrated in the flexible member of the air spring. The flexible member is clamped between the end members of the air spring and when the end members move toward each other in the longitudinal direction, the length of the electrically conductive helix changes and thereby its inductivity. A conclusion can be drawn from the magnitude of the inductivity as to the distance of the end members of the air spring from each other and therefore as to the level of the vehicle in the region of the air spring. The elevation sensor, which is disclosed in German patent publication 4,413,559, is for air springs and includes a simple configuration and a large measuring range. It has, however, been shown that the electrical helix integrated into the flexible member exhibits a small inductivity so that changes thereof can only be measured with a complex measuring arrangement.
It is an object of the invention to provide an air spring having an integrated simply configured elevation sensor which has a large measuring range. It is a further object of the invention to provide such an air spring wherein the signal of the elevation sensor can be easily evaluated.
The air spring of the invention defines a longitudinal axis and includes; a first end member in the form of a cover; a second end member in the form of a roll-off piston at a distance from the first end member; a flexible resilient member clamped between the first and second end members so as to permit the first and second end members to move toward each other in the direction of the longitudinal axis whereby the distance changes during the operation of the air spring; an elevation sensor including an electrically conductive helix arranged in the direction of the longitudinal axis; the helix having a plurality of turns and being arranged in the air spring so that the helix changes in length when the end members move towards each other in the direction of the longitudinal axis; the first and second end members and the flexible resilient member conjointly defining an interior space; the elevation sensor further including a length-nonchanging element mounted in the interior space on one of the end members so as to be in electrical interaction with a number of the turns; and, the number of the turns being dependent upon the distance between the end members.
The advantage of the invention, which is achieved therewith, is especially seen in that the elevation sensor supplies an electrical signal which is easy to evaluate while providing a simple configuration and a large length measuring range. This is so because a portion of the turns of the length-changing electrical helix enters into electrical interaction with the element not changing in length (length-nonchanging element) and, in this way, adequately large electrical signals are generated which are accessible to a simple evaluation. A further advantage of the invention is that the sensor has a low susceptibility to disturbance because the electrical measurement signals are energy rich.
According to a feature of the invention, the element is mounted in a recess disposed in one of the end faces of an end member. The advantage of this embodiment is that for an intense spring deflection of the air spring, the length-changing element cannot be damaged.
The element which does not change in length can, for example, be a core of ferromagnetic material Preferably, the element, which does not change with respect to its length, is, however, configured as an electrically conductive coil. The coil and the length-changing helix are preferably so arranged that their longitudinal axes run substantially parallel to each other. The advantage of this embodiment is that an electrical current can be driven through the coil in a controlled manner and, via the current, a magnetic field is generated in the vicinity of the coil. This magnetic field generates, in turn, a voltage in the length-changing electric-conductive helix which there leads to a current flow. The magnitude of the induced voltage, and therefore the magnitude of the current flow, is dependent upon the number of turns of the length-changing helix which are disposed in the effective region of the coil.
Basically two possibilities are present for evaluating the voltage induced in the length-changing helix. The first possibility is that an alternating current voltage having constant amplitude is applied to the coil. An alternating-current voltage is induced in the length-changing helix with an amplitude depending upon the spring deflection. This amplitude is evaluated, for example, with the aid of an envelope demodulator. A second possibility provides impressing a voltage pulse onto the coil. The voltage pulse leads to the situation that a voltage is induced in the length-changing helix, which causes a current flow in the helix. With an evaluation circuit, the time is measured which elapses until reaching a specific current threshold value in the length-changing helix. The more turns of the length-changing helix are in the active region of the coil, the higher is the induced voltage and the faster the current in the length-changing helix increases to the threshold value. From the increase time of the current, a conclusion can be drawn directly as to the distance of the end members of the air spring. Compared to the first possibility, the second possibility affords the advantage that only short-time voltage pulses are impressed on the coil so that the elevation sensor exhibits only a low current requirement.
According to another feature of the invention, the coil has a ferromagnetic core. The advantage of this embodiment is that the ferromagnetic core conducts and therefore concentrates the magnetic field, which is generated in the vicinity of the coil.
According to another embodiment of the invention, the helix and the coil each have two electrical connections which lie in the region of the cover of the air spring. The advantage of this embodiment is that the electrical connections and the evaluation electronics are arranged in the region of the suspended mass of the motor vehicle into which the air spring is built. In this way, a simple connection of the evaluation electronics to the connectors of the helix and the coil is possible. Furthermore, fewer dynamic forces act on the cover of an air spring built into a motor vehicle than on the roll-off piston so that the connectors are mechanically better protected in the region of the cover.
According to another feature of the invention, the length-changing helix is mechanically connected at one end with the cover and at the other end with the roll-off piston of the air spring. Preferably, in this case, the helix is within the air volume enclosed by the resilient member of the air spring. Use is made of this embodiment especially when the length-nonchanging element is mounted on the cover of the air spring.
According to still another embodiment of the invention, the length-changing helix is a component of the resilient member of the air spring. The advantage of this embodiment is that the helix, which is integrated into the resilient member, is well protected against mechanical loads. A further advantage of the embodiment is that no disturbing noises are caused by the helix when there is a deflection of the air spring.
According to another feature of the invention, the slope angle of the length-changing helix increases starting from the end of the helix, which is facing toward the end member on which the length-nonchanging element is mounted, in the direction of the end of the helix which is facing toward the other end member. The advantage of this embodiment is that the linearity of the elevation sensor is improved. This can be achieved, for example, in that the turn density from one end of the helix to the other end increases. Then, the compressed helix exhibits a constant slope angle over its entire length and, in contrast, the helix exhibits the above-described contour in the expanded state.
According to another embodiment of the invention, the length-changing helix and the length-nonchanging element are arranged coaxially with respect to each other. The advantage of this feature is that an interaction, which is distributed uniformly over the space, occurs between the helix and the length-nonchanging element. The length-changing helix is especially uniformly penetrated by the magnetic field which is generated by the length-nonchanging element configured as a coil and vice versa.